Imagine a house. It would be a concrete cuboid with no support walls. It would have various door-shaped holes in it at various points at ground level. You could use those holes to install door or window modules. So you could have a normal window if the bottom of the unit you install into the standard hole is solid. Or you could have a french window. It is key that all the holes in the box be uniform.

Here is a quick floor plan:

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Side view with example holes:

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There would be some system of movable walls but these would not be structural.

The house would have no plumbing, heating, electricity, or communication wires installed as part of the design. Rather, these would run through ducts attached to the walls and ceiling that can be changed out/upgraded, or rearranged over time. The walls must all be solid, and the movable walls will have to make allowances for shifting ductwork.

If there are multiple floors or a basement access to them would be through a standard hole in the floor (which, when not in use, would be plugged with a standard hole plug module). You could attach a spiral staircase or descending ladder to allow access (or, outdoors, a tenement-style staircase for access to upper floors).

Utility access to the house would also be through a standard hole. Items like a central air conditioner or water heater could be located in a cabinet outside the box, or in the basement, with access between the basement and the other floors through pipes in ducts that go through standard holes.